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Rotation Among High Mass Stars: A Link to the Star Formation Process?

Rotation Among High Mass Stars: A Link to the Star Formation Process?. S. Wolff and S. Strom National Optical Astronomy Observatory. Initial Rotation vs Mass <v(Birthline)> ~ 0.15 v(esc). v (esc). Single formation mechanism: 0.2-30 M sun.

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Rotation Among High Mass Stars: A Link to the Star Formation Process?

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  1. Rotation Among High Mass Stars: A Link to the Star Formation Process? S. Wolff and S. Strom National Optical Astronomy Observatory

  2. Initial Rotation vs Mass<v(Birthline)> ~ 0.15 v(esc) v (esc) Single formation mechanism: 0.2-30 Msun

  3. Early Hints: Distribution of Rotational Velocities Depends on Environment • Wolff, Edwards & Preston observations of Orion B stars • 1982 paper shows that the bound ONC cluster exhibits • Much higher median rotation speed • Lack of slow rotators compared to stars distributed in the surrounding unbound association • Guthrie (1982) study of late B stars showed that on average field stars rotated more slowly than B stars in clusters

  4. Unevolved Field B Stars 4 < M/MSun < 5

  5. Unevolved B Stars in h and chi Per 4 < M/MSun < 5

  6. Cumulative Distribution of vsini MWG Clusters and Field Stars: 6-12 Msun Field Bound Clusters

  7. Cumulative Distribution of vsini MWG Clusters, Field &Associations: 6-12 Msun Associations

  8. R136The Challenge: Source Confusion

  9. Selecting the Sample

  10. R 136 Observations11 O Stars; 15 B Stars 25 Msun 12 Msun 5 Msun

  11. Key Results for B stars • R136: 15 B Stars (6-12 MSun) • Results consistent with studies of regions in Milky Way • B stars in R 136 lack cohort of slow rotators • R 136: <vsini> = 233 +- 19 km/sec • LMC Field: <vsini> = 105 +- 8 km/sec • LMC Clusters: <vsini> = 147 +- 14 km/sec

  12. Rotation at Higher Masses15-30 Msun LMC clusters (Hunter et al. 2008) R136 O stars

  13. Rotation at Higher Masses: Key Results • R136: 11 O Stars (15-30 MSun) • R 136: <vsini> = 189 +- 23 km/sec • LMC Clusters: <vsini> = 129 +- 13 km/sec

  14. Environment or Something Else? • Decrease in vsini During Main Sequence Evolution • 6-12 Msun: vsini constant during first 12-14 Myr of evolution away from ZAMS (Wolff et al.; Huang and Gies) • Metallicity • Binarity

  15. 15-30 Msun: Evolutionary Effects Appear Negligible During Most of MS Evolution x 3.2 <log g < 4 log g > 4 Hunter et al. 2008

  16. Metallicity: N(vsini): 6-12 MsunLMC and MW Appear Similar MWG Field Stars , LMC Clusters

  17. Binarity??? • Analysis includes all stars in each type of environment independent of knowledge of binary properties • Do binary properties depend on environment? • Does rotation depend on binary properties?

  18. Why should birth in a cluster or the field matter? • Nature?: Differences in star-forming core initial conditions • Nuture?: Environmental conditions (radiation field; stellar density) • We argue that differences in initial conditions dominate

  19. Physical Mechanism Responsible for RotationLow Mass Stars (Disk-locking): Ω ~ (Macc/dt)3/7 B-6/7 Star and disk ‘locked’ at the co-rotation radius where Pdyn = Pmagnetic Wdisk = Wstar

  20. Potential Effects of Environment • For a low-mass star, the lifetime of the disk plays a major role in determining rotation rate on the main sequence • Stars deposited on a “birthline” well above the ZAMS on PMS convective tracks • Stars that lose their disks will spin up more as they contract toward the main sequence and will become rapid rotators • Stars that remained locked to their disks until contraction is nearly complete will be slow rotators • Cluster environments are more conducive to early disk loss • In cluster regions containing a number of early-type stars, external uv radiation fields can erode disks rapidly via photoevaporation

  21. Observational Tests of the Effects of Environment vs Initial Conditions • Difficult for low mass stars because initial rotation speeds on birthline altered during subsequent evolution • But for typical accretion rates, stars with M > 8 Msun are already on the main sequence when the main accretion phase ends • Initial speed not altered by subsequent additional contraction • But what about variations in disk lifetime?

  22. Variations in Disk Lifetime Unlikely to Account for Distribution of O & B Star Rotation Rates • Disk lifetimes are short (t < 105 yr) • Rapid disk disruption driven by photoevaporation from the forming star • No evidence of disks among B0-B3 stars among rich, young clusters with ages t ~ 1 Myr • Photoevaporation by external sources requires much longer • In the ONC, photoevaporation by external sources of a disk of 0.1 Msun (relatively low for a B star disk) would require 106 years

  23. Rotation could reflect differences in initial conditions in star-forming core • Cluster-forming molecular clumps appear to have higher turbulent speeds (Plume et al. 1997) • If higher turbulent speeds also characterize the star-forming cores, then higher initial densities are required in order that self gravity can overcome the higher turbulent pressures • Higher core densities lead to shorter collapse times and higher high time-averaged accretion rates (McKee & Tan 2003) • In the context of ‘disk-locking’, higher time-averaged accretion rates lead to higher initial rotation speeds

  24. Needed Observations • Differences in turbulence between individual coresnot yet established • Direct measurements of infall rates are needed • Requirements: • A list of massive stars still embedded within their natal cores • Measurements of infall rates • Ultimately from ALMA • In the near-term, from high resolution mid-IR spectroscopy • The observations of BN by Kleinmann et al (1983) provide an example • 8-10 m telescopes can make a start on this problem

  25. Summary • N(vsini) differs between cluster & field • Higher median rotation in dense, cluster-forming regions • Near absence of slow rotators in cluster-forming regions • Rotation differences likely result from differences in initial conditions

  26. Summary • Initial conditions -- specifically higher turbulent speeds and resulting higher time-averaged accretion rates -- can account for differences in rotation speeds between cluster & field • Direct measurements of infall rates for individual cores in cluster- and association- forming regions will provide an important test of the ‘hints’ provided by the results of stellar rotation studies

  27. Turbulence in Clusters vs Field • Gas turbulent velocities in these regions are high (e.g. Plume et al. 1997) • High turbulent velocities lead to: • rapid protostellar collapse times and • high time-averaged accretion rates (dMacc/dt) • Conditions in dense, bound clusters should favor formation of • Stars that rotate rapidly owing to high dMacc/dt

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